Rf choke for gas delivery to an rf driven electrode in a plasma processing apparatus

ABSTRACT

In large area plasma processing systems, process gases may be introduced to the chamber via the showerhead assembly which may be driven as an RF electrode. The gas feed tube, which is grounded, is electrically isolated from the showerhead. The gas feed tube may provide not only process gases, but also cleaning gases from a remote plasma source to the process chamber. The inside of the gas feed tube may remain at either a low RF field or a zero RF field to avoid premature gas breakdown within the gas feed tube that may lead to parasitic plasma formation between the gas source and the showerhead. By feeding the gas through an RF choke, the RF field and the processing gas may be introduced to the processing chamber through a common location and thus simplify the chamber design.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/172,029 (APPM/12257) filed Jul. 11, 2008, which application claimsbenefit of U.S. Provisional Patent Application Ser. No. 60/951,028(APPM/012257L), filed Jul. 20, 2007, both of which are hereinincorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the present invention generally relate to an RF choke andgas feed tube for matching impedance in a plasma processing apparatus.

2. Description of the Related Art

As demand for larger flat panel displays continues to increase, so mustthe size of the substrate and hence, the processing chamber. As solarpanel demand increases, higher RF field is sometimes necessary. Onemethod for depositing material onto a substrate for flat panel displaysor solar panels is plasma enhanced chemical vapor deposition (PECVD). InPECVD, process gases may be introduced into the process chamber througha showerhead and ignited into a plasma by an RF field applied to theshowerhead. As substrate sizes increase, the RF field applied to theshowerhead may also correspondingly increase. With the increase in RFfield, the possibility of premature gas breakdown prior to the gaspassing through the showerhead increases as does the possibility ofparasitic plasma formation above the showerhead.

Therefore, there is a need in the art for an RF choke and gas feedthrough to reduce premature gas breakdown and parasitic plasmaformation.

SUMMARY OF THE INVENTION

In large area plasma processing systems, process gases may be introducedto the chamber via the showerhead assembly which may be driven as an RFelectrode. The gas feed tube, which is grounded, is electricallyisolated from the showerhead. The gas feed tube may provide not onlyprocess gases, but also cleaning gases from a remote plasma source tothe process chamber. The inside of the gas feed tube may remain ateither a low RF field or a zero RF field to avoid premature gasbreakdown within the gas feed tube that may lead to parasitic plasmaformation between the gas source and the showerhead. By feeding the gasthrough an RF choke, the RF field and the processing gas may beintroduced to the processing chamber through a common location and thussimplify the chamber design.

In one embodiment, an RF choke assembly includes a gas feed tubecomprising a metal and one or more ferrite elements surrounding the gasfeed tube.

In another embodiment, an apparatus is disclosed. The apparatus includesan RF power source, a gas source, and an RF choke assembly coupledbetween the power source and the gas source. The assembly includes a gasfeed tube comprising a metal. The gas feed tube may comprise a first endcoupled with the gas source, and a second end coupled with the RF powersource. The feed tube may also comprise one or more ferrite elementssurrounding the gas feed tube.

In another embodiment, gas delivery method includes flowing a gasthrough the inside of a metal tube. The metal tube may comprise a firstend coupled to a gas source and to ground, and a second end coupled withan RF power source. The method may also include flowing RF current alongthe outside of the metal tube such that the gas flowing inside the metaltube is not exposed to the RF current.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1A is a schematic cross sectional view of a PECVD apparatusaccording to one embodiment of the invention.

FIG. 1B is a schematic enlarged view of a portion of FIG. 1A.

FIG. 2A is a schematic view of an RF choke and gas feed through assemblyaccording to one embodiment of the invention.

FIG. 2B is an end view of FIG. 2A.

FIG. 3 is a schematic cross sectional view of an RF choke and gas feedthrough assembly according to another embodiment of the invention.

FIG. 4 is a schematic cross sectional view of an RF choke according toanother embodiment of the invention.

FIG. 5 is a schematic cross sectional view of an RF choke according toanother embodiment of the invention.

FIG. 6 is a schematic cross sectional view of a gas feed tube accordingto one embodiment of the invention.

FIG. 7A is a schematic cross sectional view of an RF choke coupled to aplasma processing chamber according to one embodiment of the invention.

FIG. 7B is a circuit diagram of FIG. 7A.

FIG. 8A is a schematic cross sectional view of an RF choke coupled to aplasma processing chamber according to another embodiment of theinvention.

FIG. 8B is a circuit diagram of FIG. 8A.

FIG. 9A is a schematic cross sectional view of an RF choke coupled to aplasma processing chamber according to another embodiment of theinvention.

FIG. 9B is a circuit diagram of FIG. 9A.

FIG. 10A is a schematic cross sectional view of an RF choke coupled to aplasma processing chamber according to another embodiment of theinvention.

FIG. 10B is a circuit diagram of FIG. 10A.

FIG. 11A is a schematic view of an RF choke 1100 according to anotherembodiment of the invention.

FIG. 11B is a schematic cross sectional view of the RF choke 1100 ofFIG. 11A.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures. It is contemplated that elements disclosed in oneembodiment may be beneficially utilized on other embodiments withoutspecific recitation.

DETAILED DESCRIPTION

In large area plasma processing systems, process gases may be introducedto the chamber via the showerhead assembly which may be driven as an RFelectrode. The gas feed tube, which is grounded, is electricallyisolated from the showerhead. The gas feed tube may provide not onlyprocess gases, but also cleaning gases from a remote plasma source tothe process chamber. The inside of the gas feed tube may remain ateither a low RF field or a zero RF field to avoid premature gasbreakdown within the gas feed tube that may lead to parasitic plasmaformation between the gas source and the showerhead. By feeding the gasthrough an RF choke, the RF field and the processing gas may beintroduced to the processing chamber through a common location and thussimplify the chamber design.

The invention will be illustratively described below in relation to aPECVD chamber available from AKT, a subsidiary of Applied Materials,Inc., Santa Clara, Calif. It is to be understood that the invention isequally applicable to any chamber that may require energizing a gas intoa plasma using an RF current including physical vapor deposition (PVD)chambers. It is also to be understood that the invention described belowis equally applicable to PECVD chambers, etching chambers, physicalvapor deposition (PVD) chambers, plasma processing chambers, and otherchambers made by other vendors.

FIG. 1A is a schematic cross sectional view of a PECVD apparatus 100according to one embodiment of the invention. The apparatus 100comprises a lid assembly 102 coupled with a chamber wall 108. Within theapparatus 100 a showerhead 110 may be disposed opposite a susceptor 104upon which a substrate 106 may be disposed for processing. Theshowerhead 110 may be supported by a bracket 140. The substrate 106 mayenter and exit the apparatus 100 through a slit valve 122 disposed in achamber wall 108. The substrate 106 may comprise a flat panel displaysubstrate, a solar substrate, a semiconductor substrate, and organiclight emitting display (OLED) substrate, or any other substrate. Theshowerhead 110 may comprise one or more gas passageways 112 extendingbetween a top surface 118 of the showerhead 110 and bottom surface ofthe showerhead. A plenum 114 may exist between the lid assembly 102 andthe showerhead 120. Gas introduced into the plenum 114 may be evenlydispersed behind the showerhead 110 for introduction into the processingspace 116 through the gas passages 112.

Gas may be introduced into the plenum 114 through a gas input 138. Thegas may be provided by a gas source 126. In one embodiment, the gassource 126 may comprise a processing gas source. In another embodiment,the gas source 126 may comprise a cleaning gas source. The gas maytravel from the gas source 126 through a remote plasma source 128 andcooled coupling 130 to an RF choke 132. The RF choke 132 may be coupledto a knuckle connector 136 that feeds the gas into the gas input 138. AnRF power source 124 may also be coupled with the knuckle connector 136by an RF feed 134.

Coupling the gas and the RF power through a common location may, on itsface, appear to be unsafe. However, RF current has a “skin effect” intraveling on conductive surfaces. RF current travels as close aspossible to the source driving it. Thus, RF current travels on thesurface of a conductive element and penetrates only to a certain,predeterminable depth (i.e., the skin) of the conductive element. Thepredeterminable depth may be calculated as a function of the frequencyof the RF current, the permeability of the material of the conductiveelement, and the conductivity of the conductive element. Thus, when aconductive element is thicker than the predetermined depth of the RFcurrent penetration, the RF current may not directly interact with thegas flowing therein. FIG. 1B is a schematic cross sectional view of aportion of FIG. 1A. FIG. 1B shows the path the RF current follows to theshowerhead (represented by arrows “A”) and the path through the RF choke(represented by arrows “B”).

As may be seen from FIG. 1B, the RF current flows on the outside of theRF feed, the outside of the gas input 138, the top of the lid 102, theoutside edge of the lid 102, the surface of the bracket 140 opposite theplenum 114, and finally across the bottom surface 120 of the showerhead.Once the RF current reaches the bottom surface 120 of the showerhead110, the gas may ignite into a plasma either within hollow cathodecavities that may be present within the gas passages 112 or within theprocessing space 116. The RF current may also travel along the top ofthe knuckle connector 136 and outside the RF choke 132. The longer thedistance that the RF current has to flow, the greater the impedance.Thus, for a center fed RF current, as chamber size increases, theimpedance to the showerhead also increases.

An RF choke may be used between the RF power source and the gas sourceto ensure an approximately uniform attenuation of voltage differencebetween the RF power source and the gas delivery system. The voltagedrop across the RF choke may be approximately equal to the voltage levelof the gas distribution showerhead. Additionally, the voltage drop alongthe RF choke may be substantially uniform. Therefore, the RF poweroutput from the RF power source that maintains and ignites the plasmawithin the processing chamber may be known and repeatable. The RF chokemay maximize the voltage transfer to the showerhead and thus make theimpedance of the RF source substantially equal to the impedance of theload.

FIG. 2A is a schematic view of an RF choke 200 according to oneembodiment of the invention. FIG. 2B is a cross sectional view of FIG.2A. The RF choke 200 comprises a coil 202 wrapped around a ferritematerial 204. Within the ferrite material 204, a cooling tube 206 may bedisposed. In one embodiment, the cooling tube 206 may comprise copper.One or more fins 208 may radiate out from the cooling tube 206 to extendthe cooling surfaces further out into the ferrite material 204 anddissipate heat from the coil 202. The coil 202 may wrap a plurality oftimes around the ferrite material 204. The coil 202 may be sufficientlythick to prevent penetration of the RF current into the inside of thecoil 202. One end 210 of the coil 202 may be coupled with the gas inputto the processing chamber and the other end 212 may be coupled to groundand the gas source. Thus, the RF current flows along the outside or skinof the coil 202 from the gas input to ground.

The inductance increases the further that the RF current travels. Givena sufficient distance, the inductance along the RF choke may besubstantially equal to the inductance of the showerhead. By increasingthe radius, the length, or the number of turns in the coil 202, theinductance may be increased. Additionally, the RF current, as it travelsalong the outside of the coil 202, contacts the ferrite material 204 andmay result in a high impedance. The length of the coil 202 should besufficiently short to ensure the impedance of the RF choke is notgreater than the impedance of the load to the showerhead. If theimpedance of the RF choke is greater than the impedance of the load,then the RF choke 200 may fail.

The ferrite material 204 may comprise a high frequency low-loss ferritematerial. In one embodiment, the ferrite material 204 may comprisehalf-cylinder blocks forming a compact cylindrical ferrite core. Inanother embodiment, the ferrite material 204 may comprise quartercylinder blocks forming a compact cylindrical ferrite core. The ferritematerial 204 raises the permeability and thus raises the inductance. Theferrite material 204, coupled with the increased RF path provided by thecoil 202 brings down the resonance of the RF current because of thecapacitance created by the RF choke 200. The RF choke 200 has a high RFimpedance to create an RF to ground isolation.

The ferrite material 204 increases the permeability and also theinductance. The ferrite material 204 additionally provides an additionalvoltage drop between the RF source and ground. The ferrite material 204may act as a thermal insulator and thus reduce the heat loss of the coil202.

The coil 202 may comprise aluminum and be sufficiently thick as toprevent penetration of the RF current into the inside of the coil wherethe gas may flow. In one embodiment, the coil 202 may comprise hardanodized aluminum. In another embodiment, the coil 202 may comprisestainless steel. The inner surface of the coil 202 may be resistiveagainst cleaning gases from a remote plasma source (RPS) such asfluorine and fluorine radicals. The coil 202 may have a large crosssection to permit a high gas conductance and thus a safe pressure windowfor stable RPS operation. Because the RF field does not penetrate intothe inside of the coil 202, the gas passing through the coil 202 doesnot see the RF field and thus may not ignite into a plasma. In otherwords, the inside of the coil 202 may comprise a field free region. Anycurrent that penetrates into the coil 202 or that does not dissipate bythe end of the coil and thus encounters the gas may be so low comparedto the RF current that enters the showerhead that the no plasma mayform. If a plasma forms in the RF choke 200, the amount of RF currentflowing to the RF choke 200 may increase and cause a decrease in the RFcurrent to the showerhead. In one embodiment, the ferrite elements maynot be present.

FIG. 3 is a schematic cross sectional view of an RF choke 300 accordingto another embodiment of the invention. The RF choke 300 comprises a gastube 302 having one or more fins 304 extending radially outward from thegas tube 302. In one embodiment, the fins 304 may comprise aluminum. Oneor more ferrite disks 306 may also encircle the gas tube 302 and bedisposed between fins 304. In one embodiment, the ferrite disks 306 maycomprise low-loss ferrites having half donut pairs coupled together toform electromagnetically continuous toroids. In another embodiment, theferrite disks 306 may comprise low-loss ferrites having a ring shapethat each completely encircle the gas tube 302. It is to be understoodthat other shapes of the ferrite disks 306 may be utilized. A first end308 of the RF choke 300 may be coupled to the gas input to the processchamber while a second end 310 of the RF choke 300 may be coupled toground. The RF current may travel along an RF path “C” outside of thegas tube 302 and along the fins 304. In one embodiment, the fins 304 mayextend radially from the gas tube 302 for a distance greater than thedistance than the ferrite disks 306 extend radially from the gas tube302. To accommodate high RF currents, the gas tube 302 may belengthened, more ferrite disks 306 and fins 304 may added. In oneembodiment, the distance that the fins 304 extend from the gas tube 302may be increased. In one embodiment, the RF choke 300 may be cooled bydrilling cooling channels into the gas tube 302. In one embodiment, theferrite disks 306 may not be present.

FIG. 4 is a schematic cross sectional view of an RF choke 400 accordingto another embodiment of the invention. The RF choke 400 comprises a gastube 402 separated from each other by one or more O-rings 404 extendingradially outward from the gas tube 402. In one embodiment, the O-rings404 may comprise silicon rubber. The one or more O-rings 404 may permitair to circulate around the ferrite disks 406 and cushion the ferritedisks 406 from rubbing against one another. The O-rings 404 may spacethe adjacent ferrite disks 406 apart by a predetermined distance. One ormore ferrite disks 406 may also encircle the gas tube 402 and bedisposed between O-rings 404. In one embodiment, the O-rings 404 may bereplaced by spacer elements that may create a small distance betweenadjacent ferrite disks 406. In one embodiment, an air gap may be presentbetween adjacent ferrite disks 406. In one embodiment, the ferrite disks406 may comprise a unitary material encircling the gas tube 402 andspanning a predetermined length of the gas tube 402.

In one embodiment, the ferrite disks 406 may comprise low-loss ferriteshaving half donut pairs coupled together to form electromagneticallycontinuous toroids. In another embodiment, the ferrite disks 406 maycomprise low-loss ferrites having a ring shape that each completelyencircle the gas tube 402. A first end 408 of the RF choke 400 may becoupled to the gas input to the process chamber while a second end 410of the RF choke 400 may be coupled to ground. The RF current may travelalong an RF path “D” outside of the gas tube 402. To accommodate high RFcurrents, the gas tube 402 may be lengthened and more ferrite disks 406may added. In one embodiment, the RF choke 400 may be cooled by drillingcooling channels into the gas tube 402. In one embodiment, the ferritedisks 406 may not be present.

FIG. 5 is a schematic cross sectional view of an RF choke 500 accordingto another embodiment of the invention. The RF choke 500 comprises a gasfeed tube 502 through which the processing gas may flow. A cylinderportion 504 may surround the gas tube 502. In one embodiment, thecylinder portion 504 may comprise a conductive material. In anotherembodiment, the cylinder portion 504 may comprise metal. The cylinderportion 504 may have a first end 510 and a second end 512. The first end510 may have a substantially closed end while the second end 512 maycomprise a substantially open end with one or more extensions 514extending from the first end 510. In between the extensions 514, ferritematerial 506 may be disposed. Additionally and/or alternatively, airpockets 508 may be present between the extensions 514. The RF currenttravels along the outside of the gas tube 502 a shown by arrows “E” andalong the surface of the cylinder 510 including the extensions 514. Inso doing, the impedance is increased due to the increased RF path lengthand the exposure to the ferrite material 506.

FIG. 6 is a schematic cross sectional view of a gas feed tube 600according to one embodiment of the invention. The gas feed tube 600 maycomprise an inner tube 602 that is substantially surrounded by an outertube 604. The inner tube 602 may be spaced from the outer tube 604 by anouter path 608. A cooling fluid may move through the outer path 608 asshown by arrow “G”. To permit the cooling fluid to more in a non-linearpath and thus, increase the residence time in the outer path, thecooling fluid may encounter several disturbances 606 to change the flowpath of the cooling fluid. In one embodiment, the disturbance 606 may bea wire spun around the inner tube 602. In another embodiment, thedisturbance 606 may comprise one or more flanges extending between theinner tube 602 and the outer tube 604. The processing gas may flow alongan inner path 610 within the inner tube 602 as shown by arrow “F”.

FIG. 7A is a schematic cross sectional view of an RF choke coupled to aplasma processing chamber according to one embodiment of the invention.FIG. 7B is a circuit diagram of FIG. 7A. The processing gas enters thesystem 700 through the RF choke 706 and the backing plate 702. The RFpower is supplied by the RF power source 704. The RF voltage at the gasinlet (i.e., the ground side of the RF choke 706) is zero. The RF choke706 is in parallel with the load. The RF choke 706 is deigned to have ahigh RF impedance and may be inductive or capacitive. The RF choke 706may operate at or near the resonance with or without the help ofexternal and/or stray capacitances. In terms of an equivalent electricalcircuit, the Pi network 708 may comprise the load capacitor C_(L),tuning capacitor C_(T), and the impedance of the RF choke Z_(FT). Theload impedance Z_(L) may comprise the impedance of the RF choke Z_(FT)and the impedance of the chamber Z_(CH).

FIG. 8A is a schematic cross sectional view of an RF choke coupled to aplasma processing chamber according to one embodiment of the invention.FIG. 8B is a circuit diagram of FIG. 8A. The processing gas enters thesystem 800 through the RF choke 806 and the backing plate 802. The RFpower is supplied by the RF power source 804. The RF voltage at the gasinlet (i.e., the ground side of the RF choke 806) is zero. The RF choke806 is in parallel with the load. The RF choke 806 is deigned to becapacitive with or without external capacitive loads C_(L′), C_(L″)forming the load capacitor C_(L) in the reverse L-type matching network808. In terms of an equivalent electrical circuit, the reverse L-typematching network may comprise the load capacitor C_(L), tuning capacitorC_(T), and the impedance of the RF choke Z_(FT). The load capacitorC_(L) may comprise the impedance of the RF choke Z_(FT) and an externalcapacitive load C_(L′). The RF choke 806 may be considered part of theload capacitor in the reverse L-type matching network 808.

FIG. 9A is a schematic cross sectional view of an RF choke coupled to aplasma processing chamber according to one embodiment of the invention.FIG. 9B is a circuit diagram of FIG. 9A. The processing gas enters thesystem 900 through the RF choke 906 and the backing plate 902. The RFpower is supplied by the RF power source 904. The RF choke 906 may beconsidered part of a tuning capacitor C_(T) in an L-type or Pi-matchingnetwork 908. The RF choke 906 is in series with the load. The RF choke906 is designed to be capacitive with or without external tuningcapacitors C_(T′) forming the tuning capacitor in the L-type matchingcircuit. In terms of an equivalent electrical circuit, the L-typematching network may comprise the load capacitor C_(L) and the tuningcapacitor C_(T). The tuning capacitor C_(T) may comprise the impedanceof the RF choke Z_(FT), and an external tuning capacitor C_(T′).

FIG. 10A is a schematic cross sectional view of an RF choke coupled to aplasma processing chamber according to one embodiment of the invention.FIG. 10B is a circuit diagram of FIG. 10A. The processing gas enters thesystem 1000 through the RF choke 1006 and the backing plate 1002. The RFpower is supplied by the RF power source 1004. The RF voltage at the gasinlet (i.e., the ground side of the RF choke 1006) is zero. Two RFchokes 1006 or two sections of one RF choke 1006 may be put together tobe the load and tune elements in an L-type or Pi-type matching network1008. The RF chokes 1006 are designed to be capacitive with or withoutexternal load and tuning capacitors C_(L′) and C_(T′). In terms of anequivalent electrical circuit, the network 1008 may comprise the loadcapacitor C_(L) and the tuning capacitor C_(T). The load capacitor C_(L)may comprise an external load capacitor C_(L′) and the impedance of thefirst RF choke Z_(FT1.) The tuning capacitor may comprise the impedanceof the second RF choke Z_(FT2) and an external tuning capacitor C_(T′).

FIG. 11A is a schematic view of an RF choke 1100 according to anotherembodiment of the invention. FIG. 11B is a schematic cross sectionalview of the RF choke 1100 of FIG. 11A taken along line H-H. As shown inFIGS. 11A and 11B, the ferrite elements 1104 may extend lengthwise alongthe gas tube 1102. One or more ferrite elements 1104 may be present andmay substantially cover the outer surface of the gas tube 1102.

By placing an RF choke between a gas source and a processing chamber,parasitic plasma may be reduced. The RF choke may comprise a gas tubehaving a wall thickness greater than the maximum expected penetration ofthe RF current. Additionally, the RF choke may have a sufficiently longRF path to render the impedance of the RF choke substantially equal tothe impedance of the load to the showerhead.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

1. An RF choke assembly, comprising: a gas feed tube comprising a metal;and a ferrite element coupled to and at least partially surrounding thegas feed tube.
 2. The assembly of claim 1, wherein the ferrite elementcomprises a plurality of ferrite disks.
 3. The assembly of claim 2,wherein the plurality of ferrite disks are spaced apart.
 4. The assemblyof claim 1, wherein the ferrite element comprises a plurality ofcylinders.
 5. The assembly of claim 4, wherein the plurality ofcylinders are spaced apart.
 6. The assembly of claim 1, wherein theferrite element comprises a plurality of ferrite rods.
 7. The assemblyof claim 6, wherein the plurality of ferrite rods are spaced apart. 8.The assembly of claim 1, wherein the gas feed tube comprises aluminum.9. The assembly of claim 1, wherein the gas feed tube comprises a firstinner tube and a second outer tube.
 10. The assembly of claim 9, furthercomprising one or more disturbance elements disposed between the firstinner tube and the second outer tube, the one or more disturbanceelements positioned to alter the path of cooling fluid flowing betweenthe first inner tube and the second outer tube.
 11. The assembly ofclaim 9, wherein at least one tube of the first inner tube and thesecond outer tube comprises metal.
 12. An RF choke assembly, comprising:a gas feed tube comprising a metal; and a ferrite element coupled to thegas feed tube, wherein the gas feed tube at least partially encirclesthe ferrite element.
 13. An apparatus, comprising: an RF power source; agas source; and an RF choke assembly coupled between the RF power sourceand the gas source, the assembly comprising: a gas feed tube comprisinga metal, a first end coupled with the gas source, and a second endcoupled with the RF power source.
 14. The apparatus of claim 13, whereinthe gas feed tube comprises a coil.
 15. The apparatus of claim 13,wherein the gas feed tube comprises aluminum.
 16. The apparatus of claim15, wherein the second end of the gas feed tube is coupled to ground.